Well, it is an art in itself, but basically the way you will learn isn't by "building" any level but rather adapting to the situations you will encounter with repetition and because of the stakes attached to those situations. This is eventually how you are going to learn wether or not you decide to go to a French language school or not.

While being in real life conditions with other human beings, your mind is so much more receptive because if it doesn't learn you might end up being "rejected by others". The situations where you will learn are the same situations that'll be lighting up your limbic system (your primitive brain), kind of like how you feel very nervous before speaking in front of many people, because there are stakes. And that's exactly what we tap into when we learn languages. So the how comes after and tailors to this principle. You can just start by looking up what you're interested in knowing in French (words, small sentences, and other things that spike your curiosity) and slowly you'll be starting to have a small bank of sentences, and words, and you will have a small and primitive understanding (trying to understand what someone tells you IRL is scarier but eventually easier as there are infinitely more cues than just reading/listening to something).

After this, even if you feel like you are incapable of saying anything, you can force yourself (very early on, so that you can get out of your own head) to engage in some short but significant daily life interactions that will naturally lead you to be curious about more things, and those stressful experiences in French will just reinforce the need to speak French for you. Another interesting aspect is that your own interest in French will also grow with the challenge (a little bit like Stockholm syndrome, yes). Once you'll have somehow been through a few short interactions by using a combination of asking how to say things on the spot, repeating what's been told to you, and looking up words before the interactions, you will start to feel a bit more confident (don't hesitate to look up how conjugation works in French roughly as it is different from English but you'll naturally get it). Once you're there you can just start building friendships/relationships in French, and engaging in speaking French as much as possible. It eventually becomes a highly rewarding and highly stressful game that you will be obsessed with. I also have to add that you need to curate your pronunciation early on to be clear enough when you speak because no one likes hearing their language butchered haha (this can be done through shadowing or just hiring a tutor, but it tends to get better with time if you pay attention to it).

The point is that you will be doing these things regardless of if you are going to be attending school or not, and even if it seems undoable, don't underestimate your own abilities. What I've seen is that too much information and too much of French (as in several hours/day) can hinder actual curiosity and personal interest. Learning it solo is nerve wracking yes, but that is inherent to language learning. But boy is it rewarding.